The present invention relates to a cutting tool for metal machining, comprising at least one body containing PCBN, (polycrystalline cubic boron nitride), with or without cemented carbide backing, and a hard and wear resistant refractory coating on the surface of said body. The coating is adherently bonded to the body and covers all functional parts thereof. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-crystalline xcex3-Al2O3, deposited by Plasma Activated CVD-methods (PACVD).
Cutting tools having cutting edges formed of a superhard abrasive such as a cubic boron nitride (CBN) based material are manufactured by powder metallurgical techniques and are mainly used for the machining of cast iron and hardened steel. Several types of CBN cutting tools are known, the majority of which consisting of a CBN tip that has been brazed onto a cemented carbide insert. Others have the CBN sintered directly to a cemented carbide backing of sufficient thickness to produce an insert while yet others consist of solid CBN-containing body without any cemented carbide backing.
Subjecting a sintered CBN body to temperatures over 1,000xc2x0 C. may result in undesirable structural changes in the material. Furthermore, in the case of a brazed insert the braze joint will be destroyed.
Swedish patent application 9704387-1 discloses a method for depositing refractory alumina (Al2O3) thin layers on cutting tools made of cemented carbide, cermet, ceramics or high speed steel. The invented method is a Plasma Activated Chemical Vapor Deposition (PACVD) process in which the plasma is produced by applying a bipolar pulsed DC voltage across two electrodes to which the tool substrates to be coated are fixtured and electrically connected.
WO 98/28464 discloses that by applying a coating including a MTCVD-TiCN and a CVD-Al2O3 layer to such a CBN tool material, substantial advantages in tool life can be achieved when machining hardened ball bearing steel.
U.S. Pat. No. 5,503,913 discloses improving the wear properties of tools with a cutting edge of cubic boron nitride or polycrystalline cubic boron nitride to coat the superhard body with a 0.5-6 xcexcm thick coating of one or more oxides of the metals Zr, Y, Mg, Ti or Al. The coating is deposited from the gas phase at temperatures up to 800xc2x0 C., preferably using a pulse plasma CVD-process.
According to the present invention, the disadvantages associated with the prior art are overcome.
According to one aspect of the present invention there is provided a cutting tool comprising a substrate and a coating; said substrate comprising a body containing at least 35 vol-% cubic boron nitride; and said coating comprising at least one layer of alumina, said alumina layer consisting essentially of xcex3-Al2O3 with a grain size between 5 and 200 nm.
According to a further aspect, the present invention provides a method of making a cutting tool comprising a substrate and a coating by a plasma activated chemical vapor deposition technique, wherein the technique comprises: pulsing a DC voltage between two electrodes to which substrate bodies to be coated are fixtured and electrically connected, such that the electrodes alternately act as an anode and a cathode; providing an atmosphere comprising AlCl3, H2 and Ar together with at least one oxygen donor chosen from O2, CO2, CO, and N2O; providing a pulse frequency of 5-100kHz; providing a pulse amplitude of 300-1000 volts, and providing a substrate temperature of 500-900xc2x0 C.